So for me, Koko was just 'the older-sister I played with,' and I only thought of her as a family member — not the sort of person who'd call my dad out on anything.
Literal
That-reason, for-me [gen-の] Koko [topic-は] 'play-partner honorific-older-sister' [copula-で], family [gen-の] one-person [as-としての] recognition [only-しか] not-exist-[and-く], father [to-に] challenge-[the-likes-of-ような] existence [topic-ではなかった].
A long, layered sentence. そのため '(for) that reason' as the causal opener. にとっての 'for X / from X's perspective' is the standard for framing subjective relevance. 遊び相手 'play partner / someone to play with' is a compound. としての 'in the role of X / as an X' is the formal role-specifier. 誰何する (すいかする) is a rare and literary verb meaning 'to challenge (someone's identity) / demand to know who someone is,' originally a guard's challenge — used here figuratively. ような存在 '(the kind of) existence that...' — a relative clause using 存在 'existence / being' as a grand way of saying 'kind of person.'